1. Field of the Invention
The present invention relates generally to wireless sensor technology. More particularly, the invention relates to a wireless Surface Acoustic Wave (SAW) device integrated on a flexible membrane substrate further comprising an integrated antenna and an impedance-based sensor such as a photodiode.
2. Description of the State-of-the-Art
Energy collection and storage can often limit the effectiveness of distributed wireless sensor networks. Utilization of sensor nodes equipped with either battery or power scavenging technology for long term applications can be challenging. Batteries have a finite amount of stored energy which limits the useful lifetime of the sensor. The energy landscape available to a power scavenging device can be unreliable and uncontrollable. It is possible to extend the lifetime of a sensor through low power electronics and power management (B. Warneke, B. Atwood, and K. S. J. Pister, “Smart dust mote forerunners,” in The 14th IEEE International Conference on Micro Electro Mechanical Systems, 2001, pp. 357-360; K. Aman, H. Jason, Z. Sadaf, and B. S. Mani, “Power management in energy harvesting sensor networks,” Transactions on Embedded Computing Systems, 2007, v. 6: p. 32) but the energy available at the device is still limited.
Surface Acoustic Wave (SAW) devices can offer wireless sensor nodes a locally passive method of operation in environments where battery or scavenged power is not viable. Similar to a Passive Radio-Frequency Identification (RFID) system, a SAW-based sensor system is powered by the energy of a microwave interrogation pulse. A SAW sensor connected to an antenna can receive microwave energy at one port, convert it to acoustic energy internally and then into a narrow band interrogation signal on the output port. The output signal “pings” a sensor which has varying impedance based on its state. Due to the impedance mismatch at the load and SAW device, a signal is reflected back from the load, backwards through the SAW filter, and re-radiated out the antenna. A calibrated receiver can pick up the signal and determine the state of the sensor based on the reflected signature. In this way, a sensor node deployed in the field can remain dormant and functional for long periods of time.
A number of groups have developed sensor tags that can monitor nearly any physical parameter (e.g. A. Pohl, “A review of wireless SAW sensors,” IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control, 2000, v. 47: pp. 317-332; L. Reindl, G. Scholl, T. Ostertag, H. Scherr, U. Wolff, and F. Schmidt, “Theory and application of passive SAW radio transponders as sensors,” IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control, 1998, v. 45: pp. 1281-1292; U. Wolff, F. Schmidt, G. Scholl, and V. Magori, “Radio accessible SAW sensors for non-contact measurement of torque and temperature,” in Ultrasonics Symposium, 345 E 47TH ST, NEW YORK, N.Y. 10017, 1996, pp. 359-362). For instance, A. Pohl et al., discuss a method for monitoring tire health for automotive applications in situ (A. Pohl and F. Seifert, “Wirelessly Interrogable SAW-Sensors for Vehicular Applications,” in IEEE Proceedings of the Instrumentation and Measurement Technology Conference, 1996, v. 2: pp. 1465-1468). However, the majority of the sensors that have been developed to date are based on rigid substrates and require connectors to external antennas.
To overcome these limitations, we present herein a low-profile SAW sensor tag with an integrated antenna on a flexible substrate. Specifically, we use a photodiode to wirelessly and passively sense light levels. These sensors, once fabricated, can be applied like postage stamps to flat or curved surfaces or structures. Such tags loaded with strain sensors would allow a user to monitor the structural health of buildings and bridges on a long-term basis. These devices would also allow for the near instantaneous determination of the integrity of these structures following a catastrophic event such as a fire, an earthquake, a tornado or a hurricane.